Mrs Elton has consistently included Miss Bates in her invitations (to the Weston's ball at the Crown, to Mr Knightley's garden party, and even to dinner at the vicarage!). Mrs Elton's carriage has collected Miss Bates on every occasion. ("You may be very sure I shall always take care of them"(38))

When Emma wondered how Jane Fairfax could bear "walking with the Eltons, sitting with the Eltons, spending a day with the Eltons"(33), she completely elides Miss Bates from these attentions, considering her as a conduit for gossip about the Dixons, somebody who would come over, not somebody one would press to come around when one was staying in.

Mrs Elton seems also to be cool on the Westons, because she dislikes Emma of course, but maybe also because the issue of whether Frank Churchill is a puppy or not has not yet been resolved, and maybe even some doubts about the very low marriage Mr Weston has made (Mrs Elton speaks to him but not to her).
An ill-mannered insult from Emma and to one of her proteges could not be overlooked. Miss Bates "honouring [Emma's] forbearance,in being able to pay he such attentions as she was forever receiving from [Emma and her] father" might have been a defense prompted by what Mrs Elton was contributing to the conversation.

When Mr Knightley tells Emma he fears that others would be entirely guided by your treatment of her", he might have reason to worry about Harriet or even Frank, but not about Mrs Elton.